Ink jet printing is a non-impact method for recording information in response to an electronic signal, such as that generated by a computer. In the printer the electronic signal produces droplets of ink that are deposited on a substrate such as paper or transparent film. Ink jet printers have found broad commercial acceptance due to their reliability, relatively quiet operation, graphic capability, print quality and low cost. Thermal or bubble jet drop-on-demand printers have been especially successful and have found broad application as output for personal computers in the office and the home.
Both dyes and pigments have been used as ink colorants for ink jet printers. However, dyes have several disadvantages. They are water-soluble and remain so after drying. They are redissolved by contact with water and will run when exposed to a water spill. Also dye images smear on contact with felt pen markers. In addition they exhibit poor light stability relative to pigments and are known to fade even under conditions of office fluorescent lighting. Thus, there are several disadvantages with the use of dye-based ink jet inks, many of which prohibit their use in applications requiring moisture resistance and greater light stability. Pigments can be used as ink colorants for ink jet printers. Pigment-based inks contain cosolvents which provide improved water and smear resistance and improved lightfastness compared to dyes. However, the presence of these cosolvents which include penetrants also tends to destabilize pigment dispersions. Thus, pigments are a useful alternative to dyes provided the pigment dispersions can be made stable to flocculation and settling.
Three major concerns dominate ink jet ink technology. They are (1) the drying rate, (2) print quality, and (3) reliability. Drying rate determines the throughput rate and therefore productivity.
One major deficiency of thermal ink jet printers is their relatively slow printing speed compared to printers using competing technologies. A major obstacle is the ink drying rate. In a sheet fed printer, the ink must be dry before the succeeding sheet contacts it or the ink will smear.
Drying occurs by both evaporation and penetration. Evaporation is determined by solvent vapor pressure whereas penetration is determined by interfacial energy between the ink and the paper and the porosity of the print media. The use of penetrants is known in the art as a means to increase the rate of penetration of inks. However, the known penetrants such as Butyl Carbitol.RTM., cause pigment dispersions to flocculate and therefore cannot be used in pigmented ink jet inks.
Water-based pigment dispersions are well known in the art, and have been used commercially for applying films, such as paints, to various substrates. The pigment dispersion is generally stabilized by either a non-ionic or ionic technique. Ink jet inks containing pigment dispersions stabilized with AB or BAB block copolymers are also known.
Print quality is defined by factors such as optical density, edge acuity/feathering, spray and other artifacts, dots/square inch, and drop volume. Some reliability issues are decap or crust time which is defined as the time span over which a pen can be exposed to the atmosphere without failing to print, storage stability of the ink as seen by physical or chemical changes, compatibility with equipment material, and robustness towards variability in firing conditions.
The three major concerns discussed above often compete with one another. For example, methods for decreasing drying rate will adversely affect either print quality or reliability. Using a more volatile cosolvent will decrease drying time but also decrease decap time.
Accordingly, a need exists for penetrants that will increase ink drying without destabilizing pigment-based inks.
The cosolvents or penetrants of this invention are able to increase penetration thereby decreasing drying rate without decreasing pigment dispersion stability or reduce decap time. In addition the penetrants of this invention provide maximum penetration and spreading which is highly desirable in a small drop volume ink cartridge.
A major advantage of the use of the penetrants claimed in this invention is rapid media penetration along with compatibility with aqueous pigment dispersions. The penetrants, however, are also useful in dye-based inks.